This invention relates generally to antenna elements and more particularly to small, compact antenna elements suitable for use in high gain array antennas.
As is known in the art, many installations for array antennas impose physical constraints on the size of such antennas. For example, in an aircraft pod installation each one of the antenna elements in the array thereof should have minimum depth and width. One known antenna element characterized by its relatively shallow shape is a so-called "hoghorn" antenna, described by A. B. Pippard in an article entitled "The Hoghorn -- An Electromagnetic Horn Radiator of Minimum-Sized Aperture" published in Journal of the Institute of Electrical Engineers, Vol. 93, Part IIIA, No. 1, in 1946. The hoghorn antenna as described has one offset waveguide feed structure which introduces radio frequency energy at the focal point of a parabolic wall forming the back plate of a waveguide horn. The feed structures generally used with such a hoghorn antenna include an orthogonal probe coupling or an end-on, double-ridged coaxial-to-waveguide transition section. While the hoghorn is a relatively shallow antenna element, the use of orthogonal probes in the feed structures will not readily enable such antenna element to be used in an array antenna where antenna element spacing in the order of 0.5 .lambda.m (where .lambda.m is the smallest operating wavelength in the normal operating range of the array) is required to obtain satisfactory reduction in grating lobes. Therefore, in order to provide an intermeshing of antenna elements, adjacent ones being used respectively in port and starboard array antennas, each antenna element must have a thickness less than the order of 0.25 .lambda.m. Further, the use of the end-on double-ridged transition section is generally relatively difficult and expensive to construct, especially if the element is to be operated in the 7-17 GHz frequency spectrum.